1. Field of the Invention
The present invention relates to a radio wave receiving apparatus equipped with an antenna and a tuning member.
2. Description of Related Art
There has conventionally been a communication apparatus which automatically adjusts a frequency characteristics of a tuning circuit connected to an antenna so as to highly efficiently transmit/receive a radio wave signal of a predetermined frequency (for example, Japanese Patent Application Laid-Open Publication No. 2000-231609, and U.S. Pat. No. 6,177,846).
When adjusting the tuning circuit at the time of receiving in such communication apparatus, it is necessary to perform tuning processing while receiving actual transmission radio wave, or to prepare an oscillator for generating a signal of same frequency as that of actual transmission radio wave to perform tuning processing while receiving the signal from the oscillator. In the tuning processing, the tuning circuit is adjusted so that a signal level of a detected output becomes maximum or that an AGC (Auto Gain Control) signal becomes a minimum gain control, by changing capacitance value of the tuning circuit while monitoring the signal level of the detected output obtained by receiving processing or the AGC signal.
In addition, Japanese Patent Application Laid-Open Publication No. 2000-231609 discloses a technique to adjust the tuning circuit by changing the capacitance value while a transmission circuit operates and compare signal phases of both antenna terminals so that transmission radio wave becomes strongest, as automatic adjusting processing of the tuning circuit when transmitting radio wave.
When adopting a system which receives actual transmission radio wave for performing the tuning processing in order to automatically adjust the tuning circuit in the radio wave receiving apparatus, there is a problem that the automatic adjustment of the tuning circuit can not be normally performed if the tuning circuit does not exist in the environment that the transmission radio wave can be successfully received.
On the other hand, in a system which generates pseudo transmission radio wave by an oscillation circuit and performs the tuning processing while receiving the transmission radio wave, there is a problem that the oscillation circuit accurately oscillating at same frequency as that of the transmission radio wave needs to be prepared individually. When the oscillation circuit is provided inside the receiving apparatus, because a crystal oscillator or the like needs to be used for obtaining an accurate oscillation frequency, even if the reception circuit and the like are integrated, it is difficult to mount all of the oscillation circuit on the integrated circuit. In addition, a configuration which uses a Phase Locked Loop (PLL) circuit to generate an oscillation signal of a predetermined frequency from a reference clock of the integrated circuit may be applied, but a problem that also this configuration needs to prepare an oscillation circuit individually would remain.